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[ultrafxr]

In discussion about the speed reading shown on gps units a couple friends contend that it is not always correct.  Their reasoning is that the gps calculates speed between points and if on level ground then it is accurate.  But in hilly terrain the distance between two points can be greater than on flat terrain, the gps does not compensate for that hence an error in calculating speed - you will be going faster than indicated.  Sounds reasonable to me.  What do you think?  

[BigLew55]

In discussion about the speed reading shown on gps units a couple friends contend that it is not always correct.  Their reasoning is that the gps calculates speed between points and if on level ground then it is accurate.  But in hilly terrain the distance between two points can be greater than on flat terrain, the gps does not compensate for that hence an error in calculating speed - you will be going faster than indicated.  Sounds reasonable to me.  What do you think?  

I'm no expert, but I would think that the GPS calculates speed based on two triangulated points, so the only thing it can't easily account for is curvature (rolling hills and corners), but modern GPS calcs so frequently, that doesn't even concern me.

Conversly, I KNOW my HD speedo is ALWAYS wrong....
 

[JCZ]

I have to agree with Big Lew.  What ever the gps is doing....right or wrong....we can all be reasonably sure that it's more acurate than any guage, odometer in a Harley dash.

Otherwise, when four bikes are rolling down a freeway in a four unit formation, when you stop and ask the others what they're speedometer says, they would all say the same thing instead of four different answers.....up to 10 mph difference between the four.

Generally speaking, my bike speedometer stays with my gps up to 60 mph and then the gps gradually drops off from what the speedometer says.  So when my speedometer says 120 mph, my gps says 112 mph.  My wife's speedometer says 115 mph but her gps says 112 mph.  Which one is correct?  Maybe neither one, but I'm thinking the gps is more accurate.

[ultrafxr]

I agree with you guys.  I always use my gps' speedo and think it is pretty darn accurate.   

[Twolanerider]

Jerry, at almost any given moment your displayed current speed is going to be accurate.  On a modern unit with a quick processor that re-reads so often you'll probably never see it misdisplay your current speed.  What's more likely is that a recorded maximum speed over a session in the type of terrain you describe might have a mistake that is high.  A momentary blip that was calculated higher than what was real is quite possible and even likely.

The GPS unit pictured captured this highest historical speed over a long weekend I was playing in the hills of Arkansas and then down to northern Louisiana.  All on little roads.  The captured high speed I noticed at a gas stop on Hwy 7 in central Arkansas.  Anyone that knows that road knows there's no way we're getting close to that speed.  But there is tree canopy and steep hills that will cause the kind of calculation error you're asking about.

[ultrafxr]

I have had that experience also Don - max speeds of 120+ mph.  I think you are correct sir. 

[Twolanerider]

I have had that experience also Don - max speeds of 120+ mph.  I think you are correct sir. 

I'm down in central Oklahoma now and fiddled with the same gps unit headed down here in the boonie flatlands.  It highest recorded right now is over 330 mph!!

[Midnight Rider]

The GPS is getting data from so many satellites nowadays, I have little doubt that it is very accurate.  Since I have the TTS, I have calibrated my Speedometer to within .5mph of the GPS reading at normal cruising speeds (60-80mph).  I have a smaller, hand held unit that will show how many satellites it's picking up at any given point (so you can know how accurate your position is out in the woods), and it's generally at least a half dozen, usually more.  The only time it might have some difficulty is in extremely mountainous terrain, like the Rockies, and you're standing down in what I'd call a "holler".

[Boxman88]

My GPS was always 5 MPH slower than the speedo on the bike. I figure the GPS in more accurate than the speedo. My car if different than my bike. I guess the GPS is the correct one.

[porthole]

GPS needs a bare minimum of 3 sats to compute location.
Most units use a minimum of 4 to get a fix. Add more sats into the into the data stream and you can get altitude. The GPS system will calculate an extremely accurate clock.

Take this clock, and multiple "fixes" over time and you can calculate speed. Most GPS receivers are updating position about 60 times a second. Most recreational GPS receivers take the 60 calculations and average them out over 1 or 2 seconds. With marine units that average is usually adjustable as rapid speed changes are useless on a boat.

Unless your GPS is defective, I would take it's speed reading over any vehicle based speedometer.

As for ups and down - the speed is still be calculated, very accurately. I use NorthStar GPS units on the boat, very precise units. As the boat goes up and down the waves you can see the speed change, slowing as you climb and increasing as yo start surfing down the wave.

Some time ago Power and Motoryacht magazine took a then new on the market Garmin 276, a $20000 survey GPS unit and police RADAR units and did comprehensive speed comparisons.
Both the Garmin and the survey GPS were found to be both right on the money and far more accurate then the RADAR.

As to the max speed error, this is the answer I received some time ago, make sense to me. If you think about how accurate the fix and calculations are, even a split second of sat viewing issue could throw off the "calculated" max speed. It happens so fast the display doesn't register a change, but the history records it.

While the max-speed indicator is a reliable indication of the minimum "max speed" you have achieved since last reset it is unreliable when it comes to displaying the real maximum speed. This means you can use it to prove you have gone no faster than what it displays but you can't use it to establish a new speed record. As the satellites drift in and out of sight or you encounter some multipath interference your unit will compute a slightly different solution based on the available satellites and this difference can result in an instantaneous change in position. This change can confuse the max speed indicator into thinking you traveled rapidly to the new location. Luckily these are usually very easy to spot.


BTW, the highest max speed I have seen on my Garmin has been 375 mph.

[Twolanerider]

My little six cylinder gutless Silverado can obviously go ballistic.  'cause that's the only trajectory it could ever achieve to go this fast !